In association with dramatically increased importance of computer network communication such as the Internet, higher communication speed is demanded for computer network communication. For example, the Internet has a backbone circuit as a trunk transmission line, and it becomes general to use an optical communication technique of broadband called “Wavelength Division Multiplexing (WDM)” for the backbone circuit.
On the side of WDM circuit of an optical interface device that connects between a WDM circuit and a client circuit, an optical demultiplexing device is disposed. The optical demultiplexing device has a function of multiplexing an optical signal from the client circuit for sending to the WDM circuit, while demultiplexing an optical signal from the WDM circuit for distributing to each optical interface device.
The optical demultiplexing device is provided with an amplifier for amplifying an optical signal in a former stage of executing demultiplexing of optical signal to recover attenuation of the optical signal having reached through the long-distance WDM circuit. The optical signal amplified in this manner is demultiplexed and input into each optical interface device.
However, when the amplified optical signal input into an optical interface device is excessively amplified, a Large Scale Integration (LSI) such as optical module possessed by the optical interface device will be burdened. Excess burden will lead abnormality of electronic parts, and, in the worst case, may result in failure of the optical interface device.
For addressing this, in the conventional art, a variable optical attenuator called Variable Optical Attenuator (VOA) is provided in the foremost stage, and control is made so that a signal level of the optical signal input into the optical interface device is reduced to an appropriate signal level.
Also in the conventional art, in the optical demultiplexing device, the VOA is provided in the later stage of the amplifier for amplifying an optical signal and in a former stage of demultiplexing of optical signal, and control is made to reduce a signal level of optical signal before demultiplexing to an appropriate signal level.
The conventional technology are exemplarily disclosed in Japanese Laid-open Patent Publication No. 2000-244417, Japanese Laid-open Patent Publication No. 2004-23295 and Japanese Laid-open Patent Publication No. 2004-297790
However, the conventional art has the following problems. To be more specific, when a signal level of optical signal increases and output potential of an optical module rises to be a predetermined voltage or higher, a signal level of the optical signal is attenuated by increase of electric current of VOA as a result of rise in base potential of transistor. When the power of the optical interface device is OFF, an electric current is not supplied to the VOA, so that attenuation of signal level of optical signal is 0 dB.
Here, assuming the case where the optical interface device is turned ON in the condition that the signal level of the optical signal is input maximally, the optical signal of the signal level exceeding the maximum reception power of the optical module is input into the optical module until the VOA functions in a steady state. Accordingly, there is a problem that the optical module may be damaged at the time of turning on the power of the optical interface device.
When an optical signal is in input break in the condition that the power is ON, attenuation of signal level of the optical signal by the VOA is 0 dB according to the control logic. Therefore, the optical signal is recovered from input break, and the optical signal is directly input to the optical module. When the signal level of optical signal is a signal level exceeding the maximum receiving power of the optical module, there is a problem that a damage may be exerted on the optical module.